1. Field of the Invention
This invention relates to a method for continuous manufacture of extruded rice products which may be readily rehydrated by immersion of the products in boiling or hot water for 5 to 10 minutes. The method includes the use of a preconditioner to partially pre-cook a rice flour or rice granules and water mixture, and also includes the use of an extruder wherein the rice and water mixture is advanced first through a cooking zone, then through a venting zone and a forming zone, and finally through an extrusion die to yield the products. The mixture is exposed to a minimum of mechanical shear within the extruder by directly injecting steam into the mixture and then removing a significant amount of moisture from the mixture in the venting zone. The extruded products once dried exhibit the characteristics such as good product integrity after rehydration and good tolerance to overcooking without formation of either a slimy or sticky outer surface.
2. Description of the Prior Art
Rice is a well known and important grain, being a staple and primary food for about one-half of the population of the world. However, whole rice grain, whether cooked with hull intact or initially polished to remove the hull, must be immersed for approximately 30 to 40 minutes in boiling water in order to gelatinize the starch and transform the grain into an edible state. Precooked rice, such as parboiled rice, cooks to an edible state in about fifteen minutes. In addition, rice grains for parboiling which have been broken into pieces are undesirable in the package or container and thus are typically separated out and sold at a discount as granules or for rice flour.
Increased attention in recent years has been directed in more technically advanced societies toward quick cooking rice products which can be conveniently rehydrated in hot or boiling water within a time period of five to ten minutes. Advantageously, such products may also be mixed with other foods and cooked in a microwave or conventional oven without the need for precooking the rice on a stove and thereafter draining any excess water. Furthermore, it is known that grinding whole rice grain into rice flour for manufacture of quick cooking rice enables a higher percentage of the grain to be utilized in comparison to the usuable percentage of grain that is available after, for example, polishing of the rice for transformation into white rice for cooking by the consumer.
In general, the known processes for manufacture of quick cooking rice include the steps of mixing a rice and water mixture in an extruder and raising the temperature of the mixture during advancement of the same along the length of the extruder in order to gelatinize the rice starch. Unfortunately, known processes impose an excessive amount of shear on the rice and water mixture within the extruder which causes the rehydrated product to have a sticky surface and agglomerate in any unsatisfactory manner when served. An excess of mechanical shear also reduces tolerance of the extruded product to overcooking which in some cases causes the rehydrated rice to have an unattractive, slimy outer surface, or become mushy and full apart.
Excessive shear of the rice and water mixture in the extruder can, under somme circumstances, decrease product integrity which is measured by the ability of the rice product to spring back to its original configuration after rehydration. An extreme amount of shear also causes a sliminess or stickiness characteristic that results in poor quality. Also, an extreme amount of shear can lead to increased extruder energy consumption and a variety of mechanical problems due to greater wear at elevated temperatures on the extruder components.
Hence, it would be desirable to provide a process for manufacturing quick cooking rice in such a fashion that mechanical shear imposed upon the rice and water mixture is minimized in order to improve the characteristics of the extruder product as well as to reduce energy consumption and the cost of extruder maintenance. Desirably, the process would enable the use of relatively high amounts of moisture during cooking in order to reduce shear, while ensuring that the extruded product is completely cooked before advancing toward a drying station where the products are dried during relatively short periods of time with air at ambient temperatures or temperatures somewhat above ambient.